Add Schemas guide to documentation sidebar

The schemas guide was available at /guides/schemas but wasn't linked
in the sidebar navigation. Added it to the Guides section between
Mutations and Error Handling for better discoverability.

Author: claude

docs/config.json

@@ -81,6 +81,10 @@
           "label": "Mutations",
           "to": "guides/mutations"
         },
+        {
+          "label": "Schemas",
+          "to": "guides/schemas"
+        },
         {
           "label": "Error Handling",
           "to": "guides/error-handling"